In the past, cathode ray tubes (CRT's) have dominated the electronic display market. Recently, efforts have been made to develop flat-panel display technologies, including electroluminescent, gas plasma and liquid crystal displays.
Numerous processes are known for the modulation of visible light. Among these processes, electrochromic techniques use the reversible change of color and/or optical density obtained by an electrochemical redox reaction of an electrochromic material in which the oxidized form and reduced form are of different colors, different indexes of refraction and/or different optical densities. Electrochromic materials change their optical properties due to the action of an electric field and can be changed back to the original state by a field reversal. In most electrochromic materials, the mechanism underlying the optical change is the insertion of ions into the electrochromic material and the subsequent extraction of the ions. The devices can show open circuit memory, i.e., voltage has to applied only when the optical properties are to be altered. Most electrochromic devices require an ion-containing material (electrolyte) in proximity with the electrochromic layer as well as transparent layers for setting up a distributed electric potential.
Recently it has become evident that electrochromism occurs in numerous transition metal oxides and organic materials [See C. M. Lampert and C. G. Granquist, "Introduction to Chromogenics", Large-Area Chromogenics: Materials and Devices for Transmittance Control, SPIE Institute Series Vol. IS 4, pp 2-19 (1990)]. Specific prior art electrochromic materials include metal oxides such as WO.sub.3, MO.sub.3, V.sub.2 O.sub.5, Ir.sub.2 O.sub.3 and Nb.sub.2 O.sub.5, polymers such as polyaniline, polyacetylene, polypyrrole and polythiophene, and aqueous solutions of metal ions such as Zn.
Many applications exist for electrochromic materials, including display panels, variable transmittance windows and variable reflectance mirrors. The development of flat displays and the like has been hampered by the need for transparent, electrically conductive windows. Conventionally, thin layers of metals or semiconductor oxides have been deposited on a transparent substrate such as glass to form a substantially transparent electrode. However, such designs suffer from several drawbacks. For example, in the case of conductive metal layers, optical loss is inherent and increased transmittance can only be attained by using thinner films. Such thin films, however, possess increased electrical sheet resistance and decreased current handling capacity, resulting in slower switching speeds of the displays. In addition, metal layers may undergo deleterious reactions with electrochromic materials during operation. Transparent semiconductor oxide layers also suffer from drawbacks such as relatively high electrical resistivity, limited current handling capacity and brittleness. Furthermore, the preparation of such oxide layers is relatively complex, requiring the use of elaborate fabrication techniques such as chemical vapor deposition.
U.S. Pat. No. 4,009,936 issued Mar. 1, 1977 to Kasai discloses electrochromic display devices including a solid electrochromic material and a solid electrolyte. The electrochromic material is selected from tungsten oxide, molybdenum oxide, titanium oxide, vanadium oxide, cobalt tungstate, tin oxide, tellurium oxide, iron oxide, rare earth oxides, metal halides, strontium titanate, metal carbonyls, salicylidene aniline, and organic materials containing a hydrazone group, an osazone group, a semicarbazone group or a sydnone group. The electrolyte is selected from Ag.sub.7 I.sub.4 PO.sub.4, AgI, and AgI in combination with a member of the Ag.sub.4 P.sub.2 O.sub.7 series, the Ag.sub.2 WO.sub.4 series, the RbI series, the NH.sub.4 I series, the KCN series, or the C.sub.4 H.sub.8 SCH.sub.3 I series. The display devices also include a transparent electrode in contact with the electrochromic material comprising a conductive film of unspecified composition coated on a glass substrate. The devices are said to be useful for display purposes, e.g., timepieces and the like.
U.S. Pat. No. 4,448,493 issued May 15, 1984 to Matsudaira et al discloses electrochromic display devices including an electrochromic layer and a solid proton conductive layer. The electrochromic layer consists of a transition metal oxide such as WO.sub.3, MoO.sub.3, TiO.sub.2, Ir.sub.2 O.sub.3, Rh.sub.2 O.sub.3, NiO or V.sub.2 O.sub.5. The proton conductive layer comprises a mixture of acids selected from titanic, stannic, antimonic, zirconic, niobic, tantalic and silicic acid. The display devices also include a transparent electrode contacting the electrochromic material and comprising a thin film of indium oxide (In.sub.2 O.sub.3) or tin oxide (SnO.sub.2) deposited on a transparent substrate such as glass or synthetic resin. The disclosed devices are said to possess shortened response times, with speeds on the order of 1 to 10 seconds being emplemplary.
U.S. Pat. No. 4,459,035 issued Jul. 10, 1984 to Nanya et al discloses electrochromic display devices including a reduction electrochromic material and an oxidation electrochromic material separated by an ion permeable insulating layer. The reduction electrochromic material is WO.sub.3 or MoO.sub.3, while the oxidation electrochromic material is iridium hydroxide [Ir(OH).sub.n ], rhodium hydroxide [Rh(OH).sub.n ] or nickel hydroxide [Ni(OH).sub.n ]. As the ion permeable insulator, Ta.sub.2 O.sub.5, Cr.sub.2 O.sub.3 or SiO.sub.2 may be used. The display devices also include a transparent electrode contacting the reduction electrochromic material and a counter electrode contacting the oxidation electrochromic material. During operation of the device, the oxidation electrochromic material is said to function as an acceptor of protons, thereby preventing evolution of hydrogen gas on the surface of the counter electrode. The devices may be used as display panels for electronic timepieces.
U.S. Pat. No. 4,233,339 issued Nov. 11, 1980 to Leibowitz et al discloses electrochromic display devices including an electrochromic material and an electrolyte. The electrochromic material may comprise WO.sub.3 which has been partially converted from the amorphous to the crystalline form which is said to significantly increase the etch resistance of the material, thereby increasing the useful life of the device. The electrolyte may be liquid, gel, paste or solid. The display devices also include a transparent electrode in contact with the electrochromic material comprising a conductive layer, such as tin oxide, deposited on a transparent glass or plastic substrate.
Japanese Patent no. 56-109317 to Nagasawa et al, published Aug. 29, 1981, discloses electrochromic display devices having a layer of amorphous WO.sub.3 and a layer of crystalline WO.sub.3 separated from each other by an ion conductive layer such as SiO, Al.sub.2 O.sub.3, ZrO.sub.2, MgF.sub.2 or CaF.sub.2. The devices also include a transparent electrode in contact with each of the WO.sub.3 layers comprising tin oxide, indium oxide or indium tin oxide (ITO). The crystalline WO.sub.3 is said to maintain a coulomb balance within the devices and to produce long life and high reliability.
U.S. Pat. No. 4,135,790 issued Jan. 23, 1979 to Takahashi et al discloses electrochromic elements comprising a thin layer of electrochromic material and a thin layer of electron blocking material sandwiched between a pair of transparent electrodes to form a unit cell. Multiple unit cells are stacked together to form a multi-layer structure. The electrochromic material may be WO.sub.3 or MoO.sub.3. The use of multiple thin layers of electrochromic material is said to reduce the response times of the devices.
U.S. Pat. No. 4,768,865 issued Sep. 6, 1988 to Greenberg et al discloses transparent electrochromic windows using WO.sub.3 as the electrochromic material along with a layer of ion conductive material. A counter electrode in the form of a metal grid is placed in contact with the ion conductive material. The metal grid participates in a balancing half-cell reaction whereby the metal grid is oxidized or reduced in response to the electrochromic transition of the WO.sub.3. Use of the metal grid is said to allow operation of the device at lower potentials which prevents electrolysis of water and concurrent gas evolution. The devices have a response time on the order of two minutes. Similar devices are disclosed by Kuo-Chuan Ho, David E. Singleton and Charles B. Greenberg in an article: "Effect of Cell Size on the Performance of Electrochromic Windows," Proceedings of the Symposium on Electrochromic Materials, Proceedings--The Electrochemical Society, vol. 90, No. 2, pp. 349-364 (1989).
U.S. Pat. No. 4,887,890 issued Dec. 19, 1989 to Scherber et al discloses transparent electrochromic panes or foils including an electrochromic polymer layer and an electrolyte layer sandwiched between two transparent electrodes. Suitable polymers include polyaniline, poly-O-phenyldiamine, polyaniline-3-sulfanic acid, polypyrrole and polythiophene, while suitable electrolytes include polymeric sulfonic acid, polymeric carbonic acid, buffered H.sub.2 SO.sub.4, buffered HClO.sub.4 and HCl. Suitable transparent electrodes include In.sub.2 O.sub.3 /SnO.sub.2 (ITO), SnO.sub.2, In.sub.2 O.sub.3, Mo, Pd, Pt, Rh, Ti and ZnSe which may be coated on a glass pane or foil. The devices are said to have response times on the order of a few seconds.
U.S. Pat. No. 4,749,260 issued Jun. 7, 1988 to Yang et al discloses transparent electrochromic display devices including a layer of polyaniline electrochromic material and a layer of electrolyte material disposed between two transparent electrodes. The electrodes comprise a transparent conductive coating such as SnO.sub.2, In.sub.2 O.sub.3, Pt or Au deposited on a glass or plastic sheet. The devices may employ multiple layers of electrochromic materials to produce tint and color changes.
U.S. Pat. No. 4,550,982 issued Nov. 5, 1985 to Hirai discloses electrochromic display devices including a layer of electrochromic material and a layer of electrolyte material disposed between two transparent electrodes. The electrochromic material consists of a polymer film comprising at least one organic electrochromic material and at least one ionic material wherein the ionic material is capable of exchanging ions with the organic electrochromic material to serve as an ion donor or acceptor. Suitable electrodes include SnO.sub.2 or ITO coated on a glass or plastic plate. The devices possess a response time on the order of 0.5 to 6 seconds.
PCT published Patent Application No. WO 92/18896, which is hereby incorporated by reference, discloses an improved working electrode for display devices comprising a metal grid along with a metal oxide coating. The preferred electrochromic material comprises an aqueous solution of an electrochemically depositable metal. During operation, the metal is deposited on the working electrode to change the optical properties of the device.
The devices noted above require the use of electrodes comprising substantially transparent, electrically conductive coatings that must possess satisfactory optical properties while at the same time possessing sufficient electrical conductivity. These are competing requirements that result in less than optimal optical and/or electrical properties due to the need to maximize the combination of these properties. The present invention has been developed in view of the foregoing and to overcome other deficiencies of the prior art.